Explorers just reached the bottom of the deepest underwater cave on Earth

Explorers in the Czech Republic have just completed an expedition
to the underwater cave Hranická Propast — also known as the
Hranice Abyss — verifying for the first time since its
discovery in 1999 that it really is the deepest underwater cave
on Earth.

The team, led by Polish diver Krzysztof Starnawski, managed to
reach a depth of 404 metres (1,325 feet) using a remotely
operated vehicle (ROV), which means this limestone cavern is 12
metres (39 feet) deeper than the second deepest underwater cave
that we know of — Italy's Pozzo del Merro.

The most recent expedition, which was sponsored in part by
National Geographic, is the result of over two years of diving. Back in 2014, Starnawski
thought he had reached the bottom of the cave at a depth of 200
metres (656 feet).

As Kat Long reports for National
Geographic, with further investigation, Starnawski found
an extremely narrow 'squeeze passage' that gave way to a
gigantic, pitch black, vertical tunnel, which he tried to explore
with a probe, but ran out of line at a depth of 384 metres (1,260
feet).

In 2015, Starnawski conducted another dive in
the cave, revealing that the passage he managed to stick a probe
through last time had widened, which allowed him to actually swim
through.

While inside this black abyss, he managed to reach a depth of 265
metres (869 feet) where he unleashed another probe, finally
managing to reach something solid at a total depth of 370 metres
(1,214 feet) - possibly landing on top of fallen debris.

According to Long, during the most recent
expedition last week, the team managed to use an ROV to fully
explore the cave, verifying that it is, in fact, the deepest
known cave in the world, at a depth of 404 metres (1,325 feet).

"As the expedition leader for the last several years, I've
prepared the equipment and the route in and out for the ROV's
dive, so the ROV could go beyond the limits of a human diver, and
get through the restricted passage and between the fallen logs
and trees," Starnawski told National Geographic.

For this expedition, he dived down to 200 metres
(656 feet) to affix a guideline for the ROV to follow. His team
deployed the ROV, and he accompanied it down to 60 metres (197
feet).

"From there, the team at the surface navigated it, via
fibre-optic cable, down along my new line to 200 metres deep.
Then it went down to explore the uncharted territory — to
the record-breaking depth of 404 metres," he told National Geographic.

"The ROV that reached 404 metres has a depth gauge that was
tested and certified by our state commission, so we are 100
percent sure the measurements were accurate."

While the team is sure that the cave is the deepest known in the
world, there are still many things to learn about it. It's
thought that the limestone cavern was created by bubbling
hot mineral water filled with carbon dioxide
that slowly formed a tunnel over time, but further research is
needed to fully understand its formation.